1. Field of the Invention
This invention relates to high frequency high efficiency power oscillators and more specifically to ultrasonic devices having a self-excited oscillator employing a push-pull driving circuit for producing a high frequency output from a transducer.
2. Description of the Prior Art
There are many types of oscillator circuits known in the art, many of which are specifically designed for driving a transducer, for example, a piezoelectric crystal for producing ultrasonic waves. The RCA Designer's Handbook entitled Solid-State Power Circuits, copyright 1971 by RCA Corporation, and printed in the United States in September, 1971, describes basic circuits designed for driving transducers at ultrasonic frequencies.
Beginning at page 302, the Handbook describes the basic operating characteristics of push-pull oscillator circuits. At page 616, a dual transistor push-pull oscillating circuit is described for driving a transducer that forms part of a series-tuned load circuit. However, as the operating frequency of the push-pull oscillator in FIG. 703 is determined by the feedback from the primary windings of the output transformer, variations in the fundamental frequency of the transducer is not followed by similar changes in the operating frequency of the push-pull circuit.
Another form of oscillator circuit for driving a transducer is described in Shoh, U.S. Pat. No. 3,432,691. Shoh discloses a dual switching transistor oscillator circuit for driving an electro-acoustic converter at parallel resonance. The circuit is designed such that the power dissipated in the converter remains substantially constant while the power transferred to the load may vary over wide limits. This is done substantially by inserting an inductor in series with the output of the switching transistors such that the capacitive reactance of the electro-acoustic converter is exactly matched by the inductive reactance of the series inductor. As a result, the voltage signal placed across the load is a sine wave and is applied effectively to the pure resistive component of the converter. Furthermore, both the total power input and the power delivered to the load vary in a like manner by following the changes in the mechanical resistance of the converter. However, the Shoh design does not optimize the efficiency of the oscillator circuit driving the converter. It appears that Shoh fails to provide base drive amplitude to the transistors proportional to the amplitude of the signal driving the converter. As a result, the transistors operate in the switching mode under low power requirements the same as they operate under high power requirements.
An amplifier for driving piezoelectric ceramics is shown in Kawada, U.S. Pat. No. 3,743,868. Kawada shows series feedback from a crystal driven by a single transistor amplifier circuit. Kawada does not operate at optimum efficiency as there appears to be failure to eliminate any high frequency harmonics of the fundamental frequency of the crystal from the feedback. Furthermore, Kawada does not provide current feedback to the amplifier input proportional to the load current.